2-Nitrofluorene (2NF) is activated by bacterial nitroreductases to a potent mutagen in the Ames-Salmonella mutagenesis tester strain TA 100. Tester strain TA 100 FR (a mutant strain of TA 100 selected for a deficiency in nitroreductase activity) was not mutated by 2 NF. Both TA 100 and TA 100 FR are equally sensitive to the mutagen methyl-methanesulfonate. With TA 100, N-hydroxylaminofluorene (2HAF) was about three times more potent a mutagen than 2 NF, but did not require nitroreductase for activation, since 100 FR was also sensitive to this mutagen. 2-Nitrosofluorene (2NOF) was two times more potent a mutagen than 2-HAF, and bacteria required nitroreductase activity for mutagenesis to occur. 2-Aminofluorene was not mutagenic to either bacterial strain. Glutathione and cysteamine were quite effective in blocking 2-NF, 2-HAF and 2-NOF mutagenesis, although DNA, RNA and purine bases did not affect the mutation rate. Ascorbic acid increased 2 NF mutagenesis without altering 2HAF and 2 NOF mutagenesis.